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The Chemical Enrichment History of the Fornax Dwarf Spheroidal Galaxy from the Infrared Calcium Triplet
Near infrared spectra were obtained for 117 red giants in the Fornax dwarf
spheroidal galaxy with the FORS1 spectrograph on the VLT, in order to study the
metallicity distribution of the stars and to lift the age-metallicity
degeneracy of the red giant branch (RGB) in the color-magnitude diagram (CMD).
Metallicities are derived from the equivalent widths of the infrared Calcium
triplet lines at 8498, 8542, and 8662 A, calibrated with data from globular
clusters, the open cluster M67 and the LMC. For a substantial portion of the
sample, the strength of the Calcium triplet is unexpectedly high, clearly
indicating that the main stellar population of Fornax is significantly more
metal-rich than could be inferred from the position of its RGB in the CMD. We
show that the relative narrowness of the RGB in Fornax is caused by the
superposition of stars of very different ages and metallicities. The
metallicity distribution in Fornax is centered at [Fe/H]= -0.9, with a
metal-poor tail extending to [Fe/H] ~= -2. While the distribution to higher
metallicities is less well determined by our observations, the comparison with
LMC data indicates that it extends to [Fe/H] ~ -0.4. By comparing the
metallicities of the stars with their positions in the CMD, we have derived the
complex age-metallicity relation of Fornax. In the first few Gyr, the metal
abundance rose to [Fe/H] ~ -1.0 dex. The enrichment accelerated in the past ~
1-4 Gyr to reach [Fe/H] ~ -0.4 dex. More than half the sample is constituted of
star younger than ~ 4 Gyr, thus indicating sustained recent star formation in
Fornax. These results indicate that the capacity of dwarf spheroidal galaxies
to retain the heavy elements that they produce is larger than expected.
(Abridged)Comment: 36 pages, 19 figures, to appear in Astronomical Journal, January 200